The present invention relates in general to sealing structures and in particular to an annular seal assembly adapted to be disposed about a rotatable shaft.
Many different types of seal assemblies are known in the art for providing a seal about the outer peripheral surface of a rotatable shaft. Such seal assemblies generally include an annular metallic case which supports a resilient sealing element therein. The sealing element can be formed from one or more flat annular laminae of material. A central aperture is formed through the sealing element so as to define an inner diameter which is smaller than the outer diameter of the shaft. As a result, the sealing element is deflected by the outer peripheral surface of the shaft when inserted thereabout. This deflection of the resilient sealing element causes it to exert a compressive force against the outer peripheral surface of the shaft, thereby providing a sealing engagement therewith.
The sealing element of the seal assembly must exert a sufficient sealing pressure radially inwardly against the outer peripheral surface of the shaft in order to provide a good sealing contact therewith under varying conditions of temperature, dampness, and the like. At the same time, it is important that the sealing element be formed from a relatively non-abrasive material so as not to score or otherwise damage the enclosed shaft. To be economical, the sealing element should also be resistant to wear, heat, pressure, and corrosion, as well as being simple and inexpensive in construction. By varying the materials used to form the sealing element and the sizes thereof, the performance of the seal assembly can be adjusted to suit a particular application therefor, as well as to account for cost of the product.
Fluorocarbon resins are one type of material known in the art which meet many of the foregoing requirements for the sealing element material. Polytetrafluoroethylene, better known under the commercial designation "Teflon", is a well known brand of fluorocarbon resin which is widely used as one (and sometimes the only) lamina of material in the sealing element of a rotatable shaft seal assembly. Teflon is durable, but relatively non-abrasive, making it a preferred choice for sealingly engaging the outer peripheral surface of a rotating shaft.
While Teflon performs satisfactorily in many situations, there are certain instances where a sealing element formed solely of this material will not function adequately. One such instance is when the Teflon material is relatively cold, such as occurs during the start of operation of the device within which the seal assembly is used. The relatively cold temperature of the Teflon material reduces its inherent tendency to return to its undeflected shape, thereby causing it to exert a lesser force against the outer surface of the shaft than it does when it is warmer. Another such instance is when the shaft to be sealed exhibits a relatively large amount of eccentricity when rotated at a relatively high speed relative to the seal assembly. In these instances, it has been found that the Teflon material will not follow the eccentric surface of the rotating shaft quickly enough to prevent leakage.
In an attempt to remedy these drawbacks, it has been proposed that inner diameter of the annular Teflon material simply be formed much smaller than the outer diameter of the enclosed shaft. Although such a seal assembly does exhibit improved performance characteristics, the Teflon material tends to exert too much pressure on the enclosed shaft after it has warmed up causing undue friction and premature wear. A similar situation occurs when the deflected portion of the Teflon material is further compressed against the shaft by a spring or an elastomeric material disposed thereabout. These additional compressive means are not as responsive to temperature or other conditions as is Teflon. Thus, if the combination of the sealing forces generated by the compressive means and the Teflon material exerts a sufficient force against the enclosed shaft to prevent leakage in a cold start situation, then such combined forces are excessive after the Teflon material has been warmed and, as mentioned above, exerts a greater individual sealing force.
It has also been proposed that the Teflon material be replaced with synthetic rubbers and other resilient elastomeric materials which are not affected by cold temperatures and highly eccentric shafts to the same extent as Teflon. Such materials perform well in these situations. However, resilient elastomers are not as durable as Teflon, and they often succumb to the effects of wear, heat, pressure, and corrosion. Thus, such resilient elastomers generally wear out must faster than Teflon. Other more durable materials, such as silicone, have been found to be too abrasive, causing scoring of the shaft to be sealed. Accordingly, it would be desirable to provide an improved sealing element for a rotatable shaft seal assembly which is durable, yet performs satisfactorily when subjected to relatively cold temperatures and highly eccentric shaft situations.